1. Field of the Invention
The present invention relates to impact printers, and more particularly to dot matrix line printers in which an elongated hammerbank is driven in reciprocating fashion relative to a length of print paper while magnetic hammer actuators mounted along the length of the hammerbank in association with hammer springs are selectively actuated to release the hammer springs and thereby print dots on the length of print paper.
2. History of Prior Art
Printers are known in which an elongated hammerbank undergoes reciprocating, bi-directional motion relative to a length of print paper to effect the impact printing of dots. An example of such a printer is provided by U.S. Pat. No. 3,941,051 of Barrus et al., PRINTER SYSTEM, issued Mar. 2, 1976, which patent is commonly assigned with the present application.
The Barrus et al. patent describes a dot matrix line printer having an elongated hammerbank driven in reciprocating, bi-directional fashion by a cam drive assembly. A ribbon deck mounted within the printer adjacent the hammerbank disposes a length of ink ribbon between a plurality of hammer springs mounted in spaced-apart, parallel fashion along the length of the hammerbank and an adjacent platen. A length of print paper disposed between the length of ink ribbon and the platen is stepped through the print station defined by the space between the hammerbank and the platen by a tractor drive arrangement.
The printer described in the Barrus et al. patent performs printing in dot matrix fashion. As the hammerbank is reciprocated back and forth across the print paper, various ones of the hammer springs along the length of the hammerbank are released or fired from the spring-loaded retracted positions in which they are normally held, using associated magnetic hammer actuators. As each hammer spring is released or fired, an upper free end thereof which mounts a dot printing impact tip thereon flies forward out of the retracted position so that the dot printing impact tip impacts the length of ink ribbon against the print paper to print a dot. The hammer spring then rebounds into the retracted position. Each of the magnetic hammer actuators includes a permanent magnet coupled to the upper free end of an associated one of the hammer springs through a pole piece. The permanent magnet normally holds the hammer spring in the spring-loaded retracted position in readiness for release or firing. The magnetic hammer actuator also includes a magnetic coil surrounding the pole piece and operative, when momentarily energized, to overcome the effects of the permanent magnet and release the hammer spring to print a dot on the print paper.
In hammerbanks of the type described in the previously referred to U.S. Pat. No. 3,941,051 of Barrus et al., the various hammer springs are released or fired rapidly and repeatedly as the hammerbank moves back and forth across the length of print paper in reciprocating fashion. The repeated energizing of the magnetic coils causes them to quickly heat up, making it necessary that provision be made for cooling of the coils. One arrangement for cooling the coils is described in U.S. Pat. No. 4,033,255, of Kleist et al, PRINT HAMMER ACTUATOR FOR DOT MATRTIX PRINTERS, which patent issued Jul. 5, 1977 and is commonly assigned with the present application. The arrangement described in the Kleist et al. patent utilizes cooling fins mounted on top of the magnetic coils. The cooling fins absorb heat from the magnetic coils and readily release the heat to passing air which is blown over the fins under pressure. An alternative arrangement for providing coil cooling is described in U.S. Pat. No. 4,044,668 of Barrus et al., PRINT HAMMER MECHANISM, which patent issued Aug. 30, 1977 and is commonly assigned with the present application. In the Barrus et al. '668 patent, pressurized air is directed toward the rear of the hammerbank from which it enters the interior of the hammerbank through apertures in a rear wall as well as via the opposite open ends of the hammerbank. The air within the interior of the hammerbank moves upwardly over the magnetic coils and to the outside of the printer through a filtered exit passage.
While the magnetic coil cooling arrangements described in the Kleist et al. and Barrus et al. '668 patents are effective to provide coil cooling in most instances, such schemes are limited in their adaptability to certain hammerbank configurations. This is particularly true in the case of more recent hammerbank designs in which printing speed requirements may necessitate incorporating a considerably greater number of hammer springs into a hammerbank of given size. The magnetic hammer actuators may be redesigned in order to accommodate an increased number of hammer springs which are typically of smaller size as well as more closely spaced. The resulting high performance hammerbank typically does not lend itself to cooling through use of coil-mounted fins or configurations in which air is simply passed through the interior of the hammerbank.
In hammerbanks of the type described in the previously referred to U.S. Pat. No. 3,941,051 of Barrus et al., there is a certain amount of magnetic interaction that occurs between adjacent magnetic hammer actuators due to their close physical proximity and in some cases the use of common components. A certain amount of magnetic interaction is tolerable, particularly where the performance requirements are not especially great. Where printing speeds are increased, however, and particularly where the magnetic hammer actuators must be more closely spaced in order to accommodate increased numbers of hammer springs, the problem posed by magnetic interaction becomes much more serious.
To minimize the effects of magnetic interaction between adjacent magnetic hammer actuators, a number of techniques have been utilized. One technique which is described in U.S. Pat. No. 4,280,404 of Barrus et al., PRINTER HAVING VARIABLE HAMMER RELEASE DRIVE, which patent issued Jul. 28, 1981 and is commonly assigned with the present application, involves varying the current applied to the coils in accordance with the number of hammers being simultaneously fired. In another technique which is described in U.S. Pat. No. 4,386,563 of Farb, PRINTING SYSTEM HAVING STAGGERED HAMMER RELEASE, which patent issued Jun. 7, 1983 and is commonly assigned with the present application, alternate hammer springs are fired at different points in each firing interval so that adjacent hammer springs are not fired simultaneously. While the alternative hammer firing techniques described in these patents are effective in minimizing the effects of magnetic interaction for many applications, it would be advantageous to avoid the need for such techniques and the additional equipment and operational complexities that accompany them. Ideally, hammer firing times should be independent of such considerations so that dot spacing on the paper can be infinitely variable.
Most hammerbanks of the type described in previously referred to U.S. Pat. No. 3,941,051 of Barrus et al. include a pair of elongated, generally cylindrical shafts attached to the opposite ends thereof and slidably mounted within bearings to permit the reciprocating motion of the hammerbank. Typically, such shafts are of said configuration and are secured to the opposite ends of the hammerbank such as by gluing. Despite the considerable care which is taken when gluing the shafts to the opposite ends of the hammerbank, the shafts do not always precisely align along a common axis of elongation of the hammerbank, and the strength of the resulting bond between the shafts and the hammerbank sometimes proves to be inadequate. Accordingly, the mounting shaft arrangement within hammerbanks of this type could be improved upon.
In hammerbanks of the type described in previously referred to U.S. Pat. No. 3,941,951 of Barrus et al., the hammer springs which are disposed in generally parallel, spaced-apart relation along the length of the hammerbank are secured at their lower ends to a hammer mounting surface extending along the length of the hammerbank. Typically, each hammer spring is secured to the hammer mounting surface by a relatively short screw which extends through a mounting plate, through the lower end of the hammer spring and then into a screw hole extending into the hammerbank from the hammer spring mounting surface. Because of hammerbank design considerations, the screw holes are typically of limited length, requiring that relatively short screws be used to mount the hammer springs. Frequently, debris such as dirt and oil from the surrounding parts accumulates between the tip of the screw and the back of the screw hole after the screw has been installed. A more desirable hammerbank configuration would not so limit the hammer spring mounting screw size or trap debris in the screw holes.
In hammerbanks of the type described in previously referred to U.S. Pat. No. 3,941,051 of Barrus et al., a cover assembly is mounted on the hammerbank at the interface between the hammerbank and an adjacent paper-supporting platen. The cover assembly receives a length of ink ribbon therein from the ribbon deck and holds the length of ink ribbon between the dot printing impact tips on the hammer springs and the platen-supported print paper. As the various hammer springs are fired, the dot printing impact tips extend through apertures in the cover assembly so that small portions of the length of ink ribbon may be impacted against the print paper. A lower edge of the cover assembly is typically fastenend by screws or other appropriate fasteners to a lower portion of the length of the hammerbank adjacent the lower mounted ends of the hammer springs. The opposite upper edge of the cover assembly is desirably secured in fixed relation relative to the hammerbank.
In the particular hammerbank described in U.S. Pat. No. 3,941,051 of Barrus et al., substantially the entire hammerbank structure carries magnetic flux from the various permanent magnets of the magnetic hammer actuators. Conveniently, the upper edge of the cover assembly is attached to the fixed reference provided by an upper portion of the hammerbank using the magnetic attraction present throughout substantially the entire hammerbank. However, in hammerbank configurations in which the magnetic paths are confined to much smaller areas and do not extend throughout a substantial portion of the hammerbank, other means for securing the upper edge of the cover assembly to the hammerbank must be utilized.
Accordingly, it would be desirable to provide an improved air cooling system for hammerbanks which eliminates the need for heat radiating fins on the magnetic coils and which optimizes the cooling action of moving, pressurized air within a confined space in the hammerbank. It would furthermore be advantageous to provide a hammerbank in which the magnetic interaction between closely spaced magnetic hammer actuators is minimized so that the firing of each hammer spring can occur independently of other hammer spring firings and without the need for compensatory techniques such as varied coil current or staggered firing times. It would still furthermore be advantageous to eliminate the need for separate shafts attached to the opposite ends of the hammerbank in order to mount the hammerbank for reciprocating motion. It would still furthermore be advantageous to provide a hammerbank in which hammer spring mounting screws of longer configuration can be used and without trapping debris within the backs of the screw holes. It would still furthermore be advantageous to provide alternative arrangements for securing the upper edge of the cover assembly to the hammerbank in instances where less than substantially all of the hammerbank is magnetized by the permanent magnets of the magnetic hammer actuators. It would still furthermore be advantageous to provide a hammerbank in which portions of components within the magnetic hammer actuators are configured to enhance the confinement and flow of cooling air over the magnetic coils.